This invention relates generally to digital MTI radar systems and more particularly to adaptive systems of such type which reject reflections from moving clutter, such as rain, and retain, for detection and display, reflections from moving targets such as airplanes.
It is known in the art that, in MTI radar systems of the type contemplated herein, a reference signal is generated within the receiver of such system, such signal being coherent (that is, in phase) with each pulse in a train of successively transmitted pulses of radio frequency energy. A portion of the received energy, i.e. a reflection, resulting from each transmitted pulse is processed with the reference signal to produce a corresponding video signal during each range sweep. For convenience, the video signal so produced may be considered as a composite video signal made up of reflections from stationary objects and reflections from moving objects. The portions of the video signal made up of reflections from stationary objects are in constant phase relationship with the reference signal between successive range sweeps, whereas the portions of such video signal made up of reflections from moving objects vary in phase relationship with the reference signal between successive sweeps, the rate of change of phase being related to the Doppler velocity of the object. Therefore, by comparing the video signal of a current range sweep with the video signal of at least one previous range sweep, cancellation of the portions of such video signal resulting from stationary objects may be effected.
As is also known in the art, the moving objects referred to above may be divided into two classes: Moving clutter, such as rain, and moving targets, such as airplanes. It is generally desirable that in an MTI radar system the reflections from moving clutter together with reflections from stationary objects be rejected while reflections from the moving targets be retained. In an MTI radar system which is used for detection of airplanes expected at relatively large elevation angles the cancellation process referred to above should preferably be one which is able to "adapt" to cancel the moving clutter reflections, regardless of any changes in the Doppler velocity (within relatively wide limits) of such reflections.
In known digital MTI radar systems cancellation of moving clutter is effected by first explicitly calculating the phase difference between successive digitized video signals and thereby correcting such video signals by the calculated phase difference. While such a system may be adequate in some applications, the explicit calculation of the phase difference requires relatively extensive computation. This is so especially when, to avoid "blind" Doppler velocities in a pulsed system, the pulse repetition frequency of the system is varied.